1. Field of the Invention
The present invention relates to a method of preparing an oxide superconducting film using laser ablation.
2. Description of the Background Art
A method of forming an oxide superconducting film by laser ablation with an excimer laser beam, for example, is watched with interest in the point that it is possible to easily implement a high-quality film. In general, a pulse laser beam is employed for laser ablation.
In such laser ablation, the film has generally been formed at a relatively low film deposition rate with a pulsed laser repetition rate of not more than 10 Hz. According to published literature such as Appl. Phys. Lett., Vol. 53, No. 16, pp. 1557-1559 (1988), "Epitaxial growth of YBa.sub.2 Cu.sub.3 O.sub.7-x thin films by a laser evaporation process", for example, the known maximum film deposition rate is 1200 .ANG./min. In p. 1557, the above literature describes film deposition conditions of 4 .ANG./pulse and 1500 laser pulses for 5 minutes, which result in the aforementioned film deposition rate of 1200 .ANG./min.
The maximum film deposition rate has been thus limited to 1200 .ANG./min., for example, in general laser ablation conceivably for the reasons that this film deposition rate is sufficiently higher than those in other film deposition techniques such as sputtering, vacuum deposition etc. and no higher rate is required in application to the field of electronics, and that superconducting properties of the as-deposited oxide superconducting film are reduced at a higher film deposition rate.
When an oxide superconducting film is formed by ordinary laser ablation, the film deposition rate is increased by increase of the repetition rate. However, if the film deposition rate is increased in excess of 2000 .ANG./min., for example, the oxide superconducting film is obtained as an aggregate of fine crystal grains of not more than 1 .mu.m, to develop clear grain boundaries. This leads to reduction in critical temperature and critical current density.
Up to now, therefore, it has been impossible to implement a high-quality oxide superconducting film having excellent superconducting properties at a high film deposition rate.